Various types of polyethylenes are known in the art and each type has various applications. For example, low density polyethylene is generally prepared at high pressure using free radical initiators, or in gas phase processes using Ziegler-Natta or vanadium catalysts, and typically has a density in the range of 0.916 to 0.940 g/cm3. This low density polyethylene, produced using free radical initiators, is known in the industry as “LDPE”. LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene in the same density range, i.e., 0.916 to 0.940 g/cm3, which is linear and does not contain long chain branching is also known; this “linear low density polyethylene” (“LLDPE”) may be produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), i.e., polyethylenes having densities greater than 0.940 g/cm3, and are generally prepared with Ziegler-Natta catalysts. Very low density polyethylenes (“VLDPEs”) are also known. VLDPEs can be produced by a number of different processes yielding polyethylenes having a density less than 0.916 g/cm3, typically 0.890 to 0.915 g/cm3 or 0.900 to 0.915 g/cm3.
About 67% of global LDPE demand includes film, carrying bag, and sack applications. Some examples of these applications include agricultural, multi-layer, and shrink films, as well as reinforcements for levees. LDPE, which is soft, ductile, and flexible, is additionally utilized for strong, elastic goods, such as screw caps, lids, and coatings. There remains a demand for LDPE in the global marketplace, and consequently there is a continued need for improvements that provide cost savings.
Some improvements include using a different catalyst system. For example, some work has been done to provide branched polymers having a density of 0.940 gcm−3 or less using metallocene compounds. JP2011089019A discloses a bridged metallocene in combination with a cocatalyst (a modified clay mineral, an alkyl alumoxane, or an ionized ionic compound) and an organoaluminum compound for olefin polymerization which can produce a polyolefin which possesses long chain branching, with high activity.
JP2011137146 discloses a manufacturing method for an ethylenic polymer using a catalyst for polymerization composed essentially of a component (A): a metallocene compound represented by a specified formula; a component (B): a compound to react with the metallocene component (A) to form cationic metallocenes; and a component (C): a fine particle carrier to give ethylene polymers characterized by (i) the existence of inflection points due to strain hardening in double-logarithmic plots of elongational viscosity [η(t); Pa·s; measured at 170° and elongational strain rate [λmax; defined as ηmax(t1)/ηlinear(t1); ηmax(t1)=maximum elongational viscosity after strain hardening; ηlinear(t1)=approximate line of elongational viscosity before hardening]≧2.0. Silica, dimethylsilylene(cyclopentadienyl)(indenyl)zirconium dichloride and methyl alumoxane were reacted to give a solid catalyst, which was used for polymerization of ethylene and 1-hexene in the presence of trimethylaluminum to give a copolymer showing λmax of 17.1.
Accordingly, there is a need for new processes to produce branched polymers having a density of 0.940 g/cm3 or less. More specifically, there is a need for new catalyst systems, particularly metallocene catalyst systems to produce branched polymers having a density of 0.940 g/cm3 or less. It is further desirable that these new metallocene catalyst systems are robust and have high productivity, particularly in gas phase polymerization processes.